An accumulator type fuel injection system is known which is adapted to stably supply a high-pressure fuel stored in an accumulator to each cylinder of a diesel engine, and in a broader operational range. However, when a fuel injection rate immediately after the starting of fuel injection is excessively high in such a fuel injection system, sudden explosion combustion is carried out in an initial stage of combustion. Consequently, not only the noise of operation of the engine but also the NOx content of an exhaust gas increases.
To eliminate such inconveniences, an accumulator type fuel injection system adapted to inject a fuel at a lower fuel injection rate in an initial stage of each fuel injection cycle has been proposed. The fuel injection system relating to this proposal is provided with, for example, a low-pressure accumulator adapted to store a low-pressure fuel therein, a high-pressure accumulator adapted to store a high-pressure fuel therein, a change-over valve communicated selectively with the low-pressure accumulator or high-pressure accumulator with an injector (fuel injection nozzle) to switch an injection rate, and a switch valve adapted to communicate and shut off a control chamber of the injector and a fuel tank with and from each other and thereby control the injection time.
Regarding the formation of a fuel pressure in an accumulator, there is, for example, an accumulator type fuel injection system adapted to obtain low-pressure and high-pressure fuels by using low-pressure and high-pressure pumps driven by an engine, and an accumulator type fuel injection system adapted to obtain a high-pressure fuel by a high-pressure pump, and a low-pressure fuel by regulating the pressure of the high-pressure fuel introduced into a low-pressure accumulator (Japanese Patent Laid-Open No. 93936/1994).
In the accumulator type fuel injection system disclosed in International Patent Laid-Open WO98/09068 and adapted to obtain a low-pressure fuel in a low-pressure accumulator from a high-pressure fuel in a high-pressure accumulator, a fuel passage connecting a change-over valve and an injector together is filled with a low-pressure fuel by closing, for example, an injection time controlling switch valve and an injection rate switching change-over valve, and the low-pressure fuel is supplied to a control chamber of the injector communicating with the fuel passage to keep the injector closed. When the injection starting time comes, the switch valve is opened to discharge the low-pressure fuel in the control chamber to a fuel tank, whereby the injector is opened to carry out initial low-pressure injection (which will hereinafter be referred to as low-pressure injection). When a low-pressure injection period passes, the change-over valve is opened to inject the high-pressure fuel, which is supplied from the high-pressure accumulator, from a nozzle and carry out main high-pressure injection (which will hereinafter be referred to as high-pressure injection). When the injection finishing time has comes, the change-over valve is closed. In the low-pressure accumulator, the pressure of the high-pressure fuel which has flowed from the fuel passage thereinto is regulated to obtain a low-pressure fuel.
The target pressures of the high-pressure fuel in the high-pressure accumulator and the low-pressure fuel in the low-pressure accumulator are regulated variably in accordance with the operating condition of the engine, and low-pressure injection and high-pressure injection which suit the operating condition of the engine are executed.
In such an accumulator type fuel injection system, the formation of fuel pressure in the low-pressure accumulator is carried out by utilizing the high-pressure fuel in the high-pressure accumulator, so that the formation of fuel pressure therein tends to delay during a transitional operation of the engine in which the operating condition of the engine suddenly changes by, for example, a sudden accelerator pedal stepping operation. Therefore, a change in level of an actual fuel pressure in the low-pressure accumulator delays with respect to a target level when the engine enters into such a transitional operating condition. Namely, a low-pressure fuel suiting a suddenly changing operating condition of the engine cannot be obtained, so that low-pressure injection cannot be carried out properly in some cases. In such a case, the fuel consumption and exhaust gas characteristics may be deteriorated.
When the fuel pressure in the high-pressure accumulator is reduced at the transitional time, for example, when the engine load condition is changed from a high-load condition to a low-load condition by returning an accelerator, reducing the fuel pressure in the high-pressure accumulator to a set level in a short period of time by reducing a response delay to as great an extent as possible is preferable for improving the exhaust gas characteristics and fuel consumption performance.